1. Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method of performing random access in a wireless communication system.
2. Discussion of the Background Art
Third generation partnership project (3GPP) mobile communication systems based on a wideband code division multiple access (WCDMA) radio access technology are widely spread all over the world. High-speed downlink packet access (HSDPA) that can be defined as a first evolutionary stage of WCDMA provides 3GPP with a radio access technique that is highly competitive in the mid-term future. However, since requirements and expectations of users and service providers are continuously increased and developments of competing radio access techniques are continuously in progress, new technical evolutions in 3GPP are required to secure competitiveness in the future. Reduction of cost per bit, increase of service availability, flexible use of frequency bands, simple structure and open interface, proper power consumption of a user equipment (UE), and the like are defined as requirements.
In general, there are one or more cells within the coverage of a base station (BS). One cell may include a plurality of UEs. A UE generally performs a random access procedure to access a network. Examples of the purposes of performing the random access procedure on the network by the UE include (1) initial access, (2) handover, (3) scheduling request, and (4) timing synchronization. These examples are only examples and thus the purposes of performing the random access procedure may vary according to a system in terms of content or the number of times of performing the procedure.
The random access procedure can be classified into a contention based random access procedure and a non-contention based random access procedure. A greatest difference between the contention based random access procedure and the non-contention based random access procedure lies in whether a random access preamble is dedicatedly assigned to one UE. In the non-contention based random access procedure, the UE uses only the random access preamble dedicatedly assigned to the UE itself, and thus contention with another UE does not occur. The contention (or collision) occurs when two or more UEs attempt the random access procedure by using the same random access preamble through the same resource. In the contention based random access procedure, there is a possibility of collision since the UEs use a random access preamble randomly selected from a plurality of candidate random access preambles.
Contention resolution is required when contention occurs in the contention-based random access procedure. In general, a timer is used for contention resolution. It is determined that contention is successful if the timer starts in the random access procedure and if a control signal is successfully received from the BS before the timer expires.
However, since various types of control signals are generally transmitted from the BS to the UE, contention resolution may be incorrectly achieved when unintentional control signals are received before the timer expires. In this case, even if the random access procedure fails, the UE may determine that the random access procedure is successful. This may result in that the UE performs incorrect uplink transmission, which leads to service delay or interference to other UEs.
The present invention provides a method capable of preventing contention between user equipments while a contention-based random access procedure is performed.